This invention relates to a display apparatus, and more particularly to a display circuit including drive circuits for display apparatus in which active elements are provided to drive the display elements of respective picture elements. The active elements are operated to display a variety of data.
A display apparatus has been developed having drive circuits formed on a substrate, in a matrix, for displaying picture elements. The drive circuits are actuated by scanning signals, so that data is displayed with LED's, LCD's EL's or fluorescent display tubes arranged in matrix form. In a display apparatus of this type, when the display elements cannot store data by themselves and it is required to display one of the images input by a time-series signal (such as a composite video television signal), as a still image on the screen formed by the displaying elements arranged in matrix form, it is necessary to temporarily store signals, corresponding to the image, in memory and to supply the stored signals to the display elements when required.
Thus, in order to display a still image, it is necessary to use display elements and a memory having a capacity corresponding to the number of picture elements.
This results in an uneconomical display apparatus having an increased number of components.
On the other hand, the drive circuit shown in FIG. 1 has been proposed for use in a two-dimensional image display apparatus having drive circuits for respective picture elements. The drive circuit is for a single picture element and includes an active element for holding data for a short period of time. A writing transistor 2 is rendered conductive (on) by a signal applied to a scanning signal line 1, so that a voltage on a video signal line 3 is temporarily held by a capacitor 4. The voltage held by the capacitor 4 is applied to the gate of a display element driving transistor 6, to set the voltage of its drain electrode 7, thereby operating a display element 8 comprising an LCD, LED, EL, fluorescent display tube or the like.
The above-described drive circuits, the number of which corresponds to the number of picture elements, are integrally formed on an insulated substrate by a film technique or by utilizing a semiconductor substrate. In order for the display apparatus to display two-dimensional data, each of the drive circuit which are formed for the picture elements must operate satisfactorily. Accordingly, if it can be determined whether or not the drive circuits operate satisfactorily before the drive circuits are connected to the display elements, then display apparatuses can be manufactured with a high yield and high efficiency, because only the operative substrates will be selected and connected to the display elements. However, in order to test the drive circuit shown in FIG. 1, it is necessary that the components of the drive circuit be assembled and that the drive circuit be connected to the display element.
In order to overcome this difficulty, Japanese Patent Application Laid-Open No. 99688/1982 provides for a drive circuit which can be inspected without connection to its display element. As shown in FIG. 2, a reading transistor 9 is connected between the video signal line 3 and the driving transistor 6. Accordingly, the drain voltage of the driving transistor 6 can be applied to the signal line 3 if a signal 10 is applied to the gate of the reading transistor 9, so that the drive circuit can be inspected without being connected to the display element 8. However, the drive circuit of FIG. 2 is disadvantageous in that, in order to provide a matrix-shaped image display, it is necessary to provide a separate memory for holding data.